Rework pg_stat_statements fetching to separate text vs stats

Since we now load one or the other (instead of both) we can invent
special purpose methods for each, avoiding a bunch of showtext checks
and making the logic easier to follow.

In passing, remove a nonsensical check for sql.ErrNoRows on the result
of db.PrepareContext. This only occurs when using QueryRow in Go, not
when querying for multiple rows.

Author: lfittl

input/full.go

@@ -69,7 +69,7 @@ func CollectFull(ctx context.Context, server *state.Server, connection *sql.DB,
 		logger.PrintError("Error setting query text timeout: %s", err)
 		return
 	}
-	ts.Statements, ts.StatementTexts, _, err = postgres.GetStatements(ctx, c, connection, true)
+	ts.Statements, ts.StatementTexts, err = postgres.GetStatementTexts(ctx, c, connection)
 	if err != nil {
 		// Despite query performance data being an essential part of pganalyze, there are
 		// situations where it may not be available (or it timed out), so treat it as a
@@ -119,7 +119,7 @@ func CollectFull(ctx context.Context, server *state.Server, connection *sql.DB,
 			logger.PrintInfo("Successfully called pg_stat_statements_reset() for all queries, next reset in %d hours", config.Features.StatementResetFrequency/scheduler.FullSnapshotsPerHour)
 
 			// Make sure the next high frequency run has an empty reference point
-			_, _, resetStatementStats, err := postgres.GetStatements(ctx, c, connection, false)
+			resetStatementStats, err := postgres.GetStatementStats(ctx, c, connection)
 			if err != nil {
 				logger.PrintError("Error collecting pg_stat_statements after reset: %s", err)
 				err = nil

input/full_1min.go

@@ -17,7 +17,7 @@ func CollectAndDiff1minStats(ctx context.Context, c *postgres.Collection, connec
 	newState := prevState
 	newState.LastStatementStatsAt = time.Now()
 
-	_, _, newState.StatementStats, err = postgres.GetStatements(ctx, c, connection, false)
+	newState.StatementStats, err = postgres.GetStatementStats(ctx, c, connection)
 	if err != nil {
 		return newState, errors.Wrap(err, "error collecting pg_stat_statements")
 	}